1. Field of the Invention
This invention relates to the field of thermophotovoltaic (TPV) direct energy conversion. In particular, the present invention relates to a high voltage, low current device called a monolithic interconnected module and a design for improving the power density and conversion efficiency.
2. Background of the Invention
Thermophotovoltaics (TPV) is a direct energy conversion technology that utilizes the infrared radiation emitted from a hot metal or ceramic radiator for conversion into electricity through absorption in a semiconductor photovoltaic device. A TPV system consists of several components: 1) energy source (e.g. combustion, solar concentrator, etc.), 2) radiator (e.g. high temperature metal or ceramic), 3) spectral control (e.g. anti-reflection coating, front surface filter or back surface reflector) and 4) photovoltaic device (e.g. single junction or monolithic interconnected module (MIM)). This invention is the improvement of the MIM device that combines the spectral control and photovoltaic device functions into a single device, resulting in increased power density and efficiency.
A MIM consists of multiple series interconnected cells on a common semi-insulating substrate that results in a higher voltage and lower current device and incorporates a back surface reflector for optical recuperation. Monolithic modules (or high voltage devices) have been investigated for many years for solar photovoltaic power production and as a means of reducing internal joule heating losses by reducing the current, simplifying array fabrication and improving array reliability. Results of these investigations are described in Borden, 14th IEEE Photovoltaic Specialists Conference, p. 554 (1980), McClelland et al., 21st IEEE Photovoltaics Specialists Conference, p. 554 (1990), Rand et al., AIP Conference Proc. 52 (1992), and Spitzer et al., 22nd IEEE Photovoltaics Specialists Conference, p. 142 (1991). These devices were grown from materials such as silicon (Si, energy bandgap of 1.1 eV) and gallium arsenide (GaAs, energy bandgap of 1.4 eV) and used the sun as the source of light for conversion into electricity.
More recently, MIMs have been grown from indium gallium arsenide (InGaAs, energy bandgaps ranging from 0.5 eV to 0.75 eV). The smaller bandgaps enable indium gallium arsenide-based devices to respond to energy with a longer wavelength than visible light, such as that emitted by a hot metal radiator. These devices employ a back surface reflector to recover radiation that is initially non-convertible. The back surface reflector is placed behind the device, directing above-bandgap energy through the cell for a second time for absorption in the active region. By using an n-doped substrate (typically indium phosphide), absorption in the substrate outside the active region can be reduced but not eliminated, as was demonstrated for conventional (non-MIM) thermophotovoltaic devices, integrated into an array using conventional solar cell interconnection technology, in U.S. Pat. No. 5,753,050 by Charache et al.
Combining the monolithic module and BSR (back surface reflector) technologies optimizes device performance. The substrate used in the MIM is semi-insulating and does not absorb non-convertible radiation. FIG. 1 is a schematic of a typical prior art MIM design with a p/n cell architecture. A p-doped emitter with an n-doped base cell architecture is traditionally chosen for the MIM to minimize electrical and optical losses. The two layers that make the greatest contributions to these losses are the p-doped InGaAs emitter and n-doped InGaAs lateral conduction layer (LCL). Measurements have indicated that p-type material has 20 times higher free carrier absorption than equivalently doped n-type material. Therefore, minimizing the thickness of p-doped material maximizes device efficiency. A trade-off between free carrier absorption, sheet resistance and active device area for, the p-doped emitter dictates that the emitter thickness be on the order of 0.3 .mu.m. However, recent calculations suggest that the maximum efficiency obtainable with a 0.3 .mu.m thick p-doped emitter is approximately 15%; therefore, this layer thickness has been reduced to 0.1 .mu.m which increases the device efficiency to 18%. As the radiator temperature is increased or the bandgap of the device is further decreased, the sheet resistance of the emitter layer begins to substantially decrease the electrical performance of the device. Therefore, with a p-doped emitter, both the power density and efficiency are inherently limited.
Two variations of the p/n MIM design have been demonstrated. The conventional MIM, described in Wilt et al., Proc. 3rd NREL Conf. On TPV Gen. Of Elect, pp. 237, AIP 401 (1997), utilizes a thick (typically, &gt;1 .mu.m), highly doped (typically greater than 10.sup.19 cm.sup.-3) LCL to conduct the current the entire length of the cell. This type of interconnect scheme is shown in FIG. 2, with the structure of the invention. Alternatively, the interdigitated MIM, described in Ward et al., Proc. 3rd NREL Conf. On TPV Gen. Of Elect, pp. 227, AIP 401 (1997) is able to use a thinner, lower-doped LCL because the current must flow only a short distance to the nearest grid finger. This type of interconnect scheme is shown in FIG. 3, with the structure of the invention. The reduction in LCL thickness and doping level in the interdigitated MIM significantly reduces the optical losses associated with the LCL, but this design does nothing to reduce the losses associated with the p-type emitter.